Formation waters are often produced concurrently with hydrocarbons. Higher amounts of produced water occur during the middle or later stage of the primary production after water breakthrough. A further increase in the amounts of produced water also occurs during the secondary treatment, in which large amounts of external saline water are injected from the surface into the reservoir formation to sustain hydrocarbons production. The amounts of produced waters in some cases could reach 90% or more of the total fluids produced.
Most produced waters are hyper-saline chloride-type that are rich in both sodium and calcium ions. The salinity of such produced waters could be 10-15 times higher than the salinity of seawater. Chloride-rich produced waters that are high in calcium are generally high in alkaline earth cations such as strontium, barium, and in some cases radium. Some of the naturally occurring isotopes of strontium (Sr-87) and barium (Ba-130 and Ba-132) are radioactive. In addition, the availability of radium in produced water suggests that the decay series of radium's isotopes is common and such water is radioactive.
As shown in FIGS. 1 and 2, radium sources in produced water are either uranium (U-238) or thorium (Th-232). U-238 and Th-232 exist in subsurface formations as immobile species whereas their daughter nuclides (radium and its isotopes) are transported to the surface via produced water. Once radium isotopes are leached from their lithological origin, they are no longer supported by their ancestors and thus they develop their own decay series.
Naturally Occurring Radioactive Materials (NORM) in produced water pose external (near any processing equipment) and internal (during maintenance or workover) radiation hazards. NORM also renders produced waters at the surface as radioactive waste streams. The cost of handling the NORM hazards is substantial (analysis, prevention if possible, treatment, transportation, storage, disposal, equipment decontamination and long-term liability).
Sine crude oil contains a large number (hundreds) of hydrocarbons, many of which are structurally undetermined or difficult to identify, the de-oiling of produced water is also an extensive and expensive process. Partial or near efficient de-oiling of produced water might be targeted by two or three conventional processing stages. In the first stage, sludge catchers (e.g., skimmers or any other basic oil separation equipment along with coalescer) are used to separate oil droplets with sizes greater than 100 microns. The second stage targets the removal of dispersed oil droplets with sizes greater than 15-20 microns. Corrugated plate separators or hydrocyclones or centrifuges or electrostatic dehydrators or induced gas flotation without chemical addition or a combination could be used in the second stage. The third (polishing) stage is used to separate oil contents with sizes of about 10 microns from produced water. Induced gas flotation with chemical addition or carbon adsorption or extraction (liquids or supercritical fluids or polymers) or filtration (microfiltration or ultrafiltration) are frequently used in the polishing stage.
Produced water is thus an unwanted waste stream that once its' brought to the surface it becomes too expensive to treat by conventional methods. The cheapest possible way to deal with produced water is probably direct disposal by re-injecting it without treatment into abandoned oil wells or dry holes or geologically selected injection wells or salt caverns. Such disposable sites, however, may not be easily available for producers (technically and/or environmentally prohibitive).
On the other hand, produced water in some areas might be too valuable to waste. However, the only way to offset the high treatment cost of produced water is to render it harmless (NORM removal) and to extract economic values (recovery of salts, usable water, and even oil) from it using innovative and cost effective methods.
Inspection of Table 1 reveals that at least three groups of inorganic species can be selectively segregated from produced water. In the first group of inorganic species, potential radioactive alkaline earth cations (strontium, barium and radium along with its decay series) in the form of sulfate can be selectively separated from produced water. Once produced water is selectively depleted of such radioactive and pyrophoric species, produced water can be used for applications such as oil-fields water injection operations, fire extinguishing and dust control in desert areas.
The second group of inorganic species may include the simultaneous or sequential separation of magnesium hydroxide (brucite) and calcium chloride (hydrophilite) from produced water as valuable commodities. The recovered brucite can be used as: (1) an agent to scrub air pollutants or to remove transition metals from aqueous streams; (2) a coagulant in water and wastewater treatment; (3) an acid neutralizer or a base stabilizer; and (4) an odor controller. The recovered hydrophilite can also be employed as a drilling fluid in oil-fields, corrosion controller, road de-icing, dust controller, concrete additive, soil additive, tire ballasting as well as other applications.
The third group of inorganic species in produced water includes sodium and potassium chloride (sylvinite). Sylvinite is a salt that consists of sodium and potassium chloride but dominated by sodium chloride. Sylvinite is the dominant salt in produced water, and thus it's partial or near complete separation from produced water will also produce partial or near complete de-ionization of produced water. Sylvinite can be used in applications such as road de-icing, animals feeding, water softening and food processing.
This patent provides innovative near zero- or zero-discharge methods that allow the conversion of produced waters as unwanted waste streams to reusable products. The first objective of this invention is to de-NORM and partially de-salt readily and sufficiently de-oiled produced water. The second objective of this invention is to de-NORM, and partially de-salt and de-ionize readily and sufficiently de-oiled produced water. The third objective of this invention is to de-oil, de-NORM, and partially de-salt and de-ionize produced water.